For Clang, not having libstdc++ and not linked with it, but it is OK because
it is not required.

It breaks the build of the shared library.

You believe there are no case other than such two case.

No, I believe there is no case where the C++ compiler will not link in
the correct STL implementation. As such, explicitly requesting it is not
needed, but breaks building with other compilers.
Let me dig for a moment to see why the configure test doesn't fail...